I'd consider it an accessibility issue - hardcoding the syntax highlighting on the backend makes it more difficult for people to change using accessibility tools. Whereas with js-based highlighting, you can probably just change the theme and get readable output.
Mainly that JS highlighting normally works with a theme variable which is much easier to change to get, e.g., a high-contrast theme than having to muck about with 10-20 different CSS styles.
Ignoring that option, working with a known set of CSS styles in the DOM is going to be easier than dealing with John Q Random's own particular set for highlighting (assuming they haven't used the same as a popular JS highlighter, of course!)
(To my shame, whatever Hugo is now using for syntax highlighting makes an awful mess - everything is "style:color#123456", no semantic information - I'll be fixing that ASAP.)
vs
antipathy - negative / "hatred"
They are claiming that the rust team uses client side JS because they don't care about the negative implications of doing so.
Aside from my pedantry, it causes computers to make more unnecessary computations and our civilization emits more carbon. It also affects me because I currently have an old CPU and it takes a few seconds for syntax highlighting to kick in. Every time I press "Back" in my browser, syntax highlighting needs to be recalculated as well.
Oh well, on the bright side I have an itch to scratch :-).